Ultraviolet radiation-induced skin tumors in a South American opossum (Monodelphis domestica)

Highly Similar Tetramerization Domains from the p53 Protein of Different Mammalian Species Possess Varying Biophysical, Functional and Structural Properties

The p53 protein is a transcriptional regulatory factor and many of its functions require that it forms a tetrameric structure. Although the tetramerization domain of mammalian p53 proteins (p53TD) share significant sequence similarities, it was recently shown that the tree shrew p53TD is considerably more thermostable than the human p53TD. To determine whether other mammalian species display differences in this domain, we used biophysical, functional, and structural studies to compare the properties of the p53TDs from six mammalian model organisms (human, tree shrew, guinea pig, Chinese hamster, sheep, and opossum). The results indicate that the p53TD from the opossum and tree shrew are significantly more stable than the human p53TD, and there is a correlation between the thermostability of the p53TDs and their ability to activate transcription. Structural analysis of the tree shrew and opossum p53TDs indicated that amino acid substitutions within two distinct regions of their p53TDs can dramatically alter hydrophobic packing of the tetramer, and in particular substitutions at positions corresponding to F341 and Q354 of the human p53TD. Together, the results suggest that subtle changes in the sequence of the p53TD can dramatically alter the stability, and potentially lead to important changes in the functional activity, of the p53 protein.

The human and animals' malignant melanoma: comparative tumor models and the role of microbiome in dogs and humans

Currently, the most progressively occurring incident cancer is melanoma. The mouse is the most popular model in human melanoma research given its various benefits as a laboratory animal. Nevertheless, unlike humans, mice do not develop melanoma spontaneously, so they need to be genetically manipulated. In opposition, there are several reports of other animals, ranging from wild to domesticated animals, that spontaneously develop melanoma and that have cancer pathways that are similar to those of humans. The influence of the gut microbiome on health and disease is being the aim of many recent studies. It has been proven that the microbiome is a determinant of the host's immune status and disease prevention. In human medicine, there is increasing evidence that changes in the microbiome influences malignant melanoma progression and response to therapy. There are several similarities between some animals and human melanoma, especially between canine and human oral malignant melanoma as well as between the gut microbiome of both species. However, microbiome studies are scarce in veterinary medicine, especially in the oncology field. Future studies need to address the relevance of gut and tissue microbiome for canine malignant melanoma development, which results will certainly benefit both species in the context of translational medicine.

Histomorphometrical study on the skin of different regions of the Caribbean common opossum (Didelphis marsupialis Linnaeus, 1758)

Common opossums (Didelphis marsupialis) are found throughout the Caribbean island of Trinidad and Tobago. The present work was conducted on the fresh normal skin of 10 common opossums and aimed to provide morphometric data and a histological description of the skin in different regions of the body. In the examined regions, the skin presented a typical thin skin morphology, with numerous folds on the surface. The dermis-epidermis junction is smooth, without the occurrence of dermal papillae. The average thickness of the opossum epidermis was 19.5 µm. The cranial region of the back showed the greatest epidermal thickness, and together with the other areas of the back presented an epidermis 2-3 times thicker compared with the other areas examined. To further confirm that the skin changes in the opossum are age- or diet-related, additional studies are required.

Spontaneously occurring melanoma in animals and their relevance to human melanoma

In contrast to other cancer types, melanoma incidence has been increasing over the last 50 years, and while it still represents less than 5% of all cutaneous malignancies, melanoma accounts for the majority of skin cancer deaths, due to its propensity to metastasise. Whilst melanoma most commonly affects the skin, it can also arise in mucosal surfaces, the eye, and the brain. For new therapies to be developed, a better understanding of the genetic landscape, signalling pathways, and tumour–microenvironmental interactions is needed. This is where animal models are of critical importance. The mouse is the foremost used model of human melanoma. Arguably this is due to its plethora of benefits as a laboratory animal; however, it is important to note that unlike humans, melanocytes are not present at the dermal–epidermal junction in mice and mice do not develop melanoma without genetic manipulation. In contrast, there are numerous reports of animals that spontaneously develop melanoma, ranging from sharks and parrots to hippos and monkeys. In addition, several domesticated and laboratory‐bred animals spontaneously develop melanoma or UV‐induced melanoma, specifically, fish, opossums, pigs, horses, cats, and dogs. In this review, we look at spontaneously occurring animal ‘models’ of melanoma and discuss their relevance to the different types of melanoma found in humans. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland..

Protective Effect of Topical Vitamin D3 against Photocarcinogenesis in a Murine Model

Background

Although the incidence of non-melanoma skin cancer is increasing, there are no effective practical preventive measures other than avoiding sun exposure.

Objective

To elucidate the protective effect of topical application of biologically active vitamin D₃ (calcitriol) on skin cancer development caused by exposure to ultraviolet (UV).

Methods

Groups of hairless mice were topically treated with either calcitriol or vehicle immediately after exposure to UVB and UVA three times weekly for the initial 20 weeks, and without UV exposure in the following 6 weeks. Tumor number was counted and biopsies were done for histopathologic analysis. The changes of cyclobutane pyrimidine dimer (CPD) were evaluated 1 hour and 11 hours after short term of UV exposure and application of calcitriol. For safety evaluation, blood test and body weights were evaluated at 23rd and 25th week.

Results

Total tumor count and number of tumors less than 3 mm in size tended to be fewer in calcitriol group, and tumors more than 3 mm in size showed significantly lower tumor formation rate in calcitriol group. Single application of calcitriol reduced CPD at 1 hour and 11 hours after UV exposure. Histopathologic analysis showed tumors with lower grade malignancy in calcitriol group which suggested a delay in tumor progression. However, serum levels of calcium and phosphate in calcitriol group were above normal range, and weight loss was found.

Conclusion

Topical calcitriol may suppress the formation and progression of UV-induced non-melanoma skin cancer by enhancing the repair mechanism of UV damage.

Preliminary genomic survey and sequence analysis of the complement system in non-eutherian mammals

The complement system is a major mediator of the vertebrate immune system, which functions in both innate and specific immune responses. It comprises more than 30 proteins working to remove foreign cells by way of anaphylatoxins, opsonins or the membrane attack complex. Over the last few years, whole genome sequences of non-eutherian mammals (marsupials and a monotreme), the gray short-tailed opossum (Monodelphis domestica), tammar wallaby (Macropus eugenii), Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus) and platypus (Ornithorhynchus anatinus), have become publicly available. Using these sequences, we have identified an array of complement components in non-eutherians using online search tools and algorithms. Of 57 complement and complement-related genes investigated, we identified 46 in the gray short-tailed opossum genome, 27 in the tammar wallaby genome, 44 in the Tasmanian devil genome, 47 in the koala genome and 40 in the platypus genome. The results of this study confirm the presence of key complement components in the immune repertoire of non-eutherian mammals and provide a platform for future studies on immune protection in young marsupials.

Bats as 'special' reservoirs for emerging zoonotic pathogens

The ongoing West African Ebola epidemic highlights a recurring trend in the zoonotic emergence of virulent pathogens likely to come from bat reservoirs that has caused epidemiologists to ask 'Are bats special reservoirs for emerging zoonotic pathogens?' We collate evidence from the past decade to delineate mitochondrial mechanisms of bat physiology that have evolved to mitigate oxidative stress incurred during metabolically costly activities such as flight. We further describe how such mechanisms might have generated pleiotropic effects responsible for tumor mitigation and pathogen control in bat hosts. These synergisms may enable 'special' tolerance of intracellular pathogens in bat hosts; paradoxically, this may leave them more susceptible to immunopathological morbidity when attempting to clear extracellular infections such as 'white-nose syndrome' (WNS).

Exponentially increasing incidences of cutaneous malignant melanoma in Europe correlate with low personal annual UV doses and suggests 2 major risk factors

For several decades the incidence of cutaneous malignant melanoma (CMM) steadily increased in fair-skinned, indoor-working people around the world. Scientists think poor tanning ability resulting in sunburns initiate CMM, but they do not understand why the incidence continues to increase despite the increased use of sunscreens and formulations offering more protection. This paradox, along with lower incidences of CMM in outdoor workers, although they have significantly higher annual UV doses than indoor workers have, perplexes scientists. We found a temporal exponential increase in the CMM incidence indicating second-order reaction kinetics revealing the existence of 2 major risk factors. From epidemiology studies, we know one major risk factor for getting CMM is poor tanning ability and we now propose the other major risk factor may be the Human Papilloma Virus (HPV) because clinicians find β HPVs in over half the biopsies. Moreover, we uncovered yet another paradox; the increasing CMM incidences significantly correlate with decreasing personal annual UV dose, a proxy for low vitamin D3 levels. We also discovered the incidence of CMM significantly increased with decreasing personal annual UV dose from 1960, when it was almost insignificant, to 2000. UV and other DNA-damaging agents can activate viruses, and UV-induced cytokines can hide HPV from immune surveillance, which may explain why CMM also occurs in anatomical locations where the sun does not shine. Thus, we propose the 2 major risk factors for getting CMM are intermittent UV exposures that result in low cutaneous levels of vitamin D3 and possibly viral infection.

UVB-induced gene expression in the skin of Xiphophorus maculatus Jp 163 B

Xiphophorus fish and interspecies hybrids represent long-standing models to study the genetics underlying spontaneous and induced tumorigenesis. The recent release of the Xiphophorus maculatus genome sequence will allow global genetic regulation studies of genes involved in the inherited susceptibility to UVB-induced melanoma within select backcross hybrids. As a first step toward this goal, we report results of an RNA-Seq approach to identify genes and pathways showing modulated transcription within the skin of X. maculatus Jp 163 B upon UVB exposure. X. maculatus Jp 163 B were exposed to various doses of UVB followed by RNA-Seq analysis at each dose to investigate overall gene expression in each sample. A total of 357 genes with a minimum expression change of 4-fold (p-adj<0.05) were identified as responsive to UVB. The molecular genetic response of Xiphophorus skin to UVB exposure permitted assessment of; (1) the basal expression level of each transcript for each skin sample, (2) the changes in expression levels for each gene in the transcriptome upon exposure to increasing doses of UVB, and (3) clusters of genes that exhibit similar patterns of change in expression upon UVB exposure. These data provide a foundation for understanding the molecular genetic response of fish skin to UVB exposure.

Recent developments in the medical and surgical treatment of melanoma

Increasing knowledge of the biology of melanoma has led to significant advances in drug development to fight this disease. Surgery is the primary treatment for localized disease and is an integral part of management in patients with more advanced disease. The last decade has become the era of targeted therapy in melanoma and has revolutionized the treatment of this disease. Since 2011, 4 new agents have been approved for the treatment of patients with metastatic melanoma: ipilimumab, vemurafenib, dabrafenib, and trametinib. Several new agents are currently in phase 3 trials with hopes of even more agents being approved for this once "untreatable" disease. How to integrate surgical options with more effective systemic therapies has become a new challenge for physicians. This review will provide an update on current surgical options, highlight the pathway to the development of the newly approved agents, and further discuss new treatments that are on the horizon.

Recent advances in sunlight-induced carcinogenesis using the Xiphophorus melanoma model

Unlike breast and prostate cancers, the nature and sequence of critical genetic and epigenetic events involved in the initiation and progression of melanoma are not well understood. A contributing factor to this dilemma, especially given our current understanding of the importance of UV light in melanoma etiology, is the lack of quality UV-inducible melanoma animal models. In this study we elaborate on the capability of UV light to induce cutaneous malignant melanomas (CMM) in Xiphophorus fishes, which were previously found to develop melanomas after acute neonatal UVB irradiation. In two separate tumorigenesis experiments, we exposed adult Xiphophorus hybrids to either acute UVB irradiations (5 consecutive daily treatments) or chronic solar irradiations (continuous UVA/UVB treatment for 9 months). Acute adult UVB irradiation resulted in the significant induction of melanomas, and moreover, this induction rate is equivalent to that of animals exposed to acute neonatal UVB irradiation. This study represents the first evidence that acute adult UVB irradiation, in the absence of any early life exposures, induces CMM. Similar to the findings conducted on other divergent melanoma models, including HGF/SF transgenic mice and Monodelphis domestica, prolonged chronic solar UV was not a factor in melanomagenesis.

Melanocytic neoplasms in a black rhinoceros (Diceros bicornis) and an Indian rhinoceros (Rhinoceros unicornis)

Melanocytic neoplasms were diagnosed in a captive black rhinoceros (Diceros bicornis) and a captive Indian rhinoceros (Rhinoceros unicornis) from different facilities. The first case was a 10-yr-old, captive-born male black rhinoceros that presented with a small firm cutaneous mass on the dorsal midline. Aspirate cytology results were suggestive of a melanocytic neoplasm, and histologic examination of the excised mass confirmed a well-differentiated neoplasm with much pigment production, minimal anaplasia, and no mitotic figures. Several months after mass removal, a similar mass with identical histologic features was excised from the right medial thigh. The second case was a 28-yr-old, wild-born female Indian rhinoceros that presented with a draining wound at the coronary band of a rear digit. Histologic examination of a biopsy from this lesion revealed a melanocytic neoplasm with moderate cellular anaplasia, frequent mitoses, and scant pigment production. At necropsy, the tumor was found to ablate P3 and most deep tissues of the toe. No evidence of vascular invasion or metastasis was found. These two cases represent the only melanocytic neoplasms in Rhinoceridae reported in detail in the literature.

The role of ultraviolet radiation in melanomagenesis

The role of ultraviolet radiation (UV) in the pathogenesis has been discussed controversially for many decades. Studies in mice (SCID, HGF/SF, SV40T) which develop malignant melanoma, show a role of UVB in melanomagenesis. In contrast to this, the role of UVA is less clear. We will review the recent in vitro and in vivo data in support of the hypothesis that UVA is also involved in the development of malignant melanoma. The role of UVA in p53 activation, apoptosis, cell cycle arrest and photoproduct formation is discussed.

Evidence of ultraviolet type mutations in xeroderma pigmentosum melanomas

To look for a direct role of ultraviolet radiation (UV) exposure in cutaneous melanoma induction, we studied xeroderma pigmentosum (XP) patients who have defective DNA repair resulting in a 1000-fold increase in melanoma risk. These XP melanomas have the same anatomic distribution as melanomas in the general population. We analyzed laser capture microdissection samples of skin melanomas from XP patients studied at the National Institutes of Health. The tumor suppressor gene PTEN was sequenced and analyzed for UV-induced mutations. Samples from 59 melanomas (47 melanomas in situ and 12 invasive melanomas) from 8 XP patients showed mutations in the PTEN tumor suppressor gene in 56% of the melanomas. Further, 91% of the melanomas with mutations had 1 to 4 UV type base substitution mutations (occurring at adjacent pyrimidines) (P < 0.0001 compared to random mutations). We found a high frequency of amino-acid-altering mutations in the melanomas and demonstrated that these mutations impaired PTEN function; UV damage plays a direct role in induction of mutations and in inactivation of the PTEN gene in XP melanomas including in situ, the earliest stage of melanoma. This gene is known to be a key regulator of carcinogenesis and therefore these data provide solid mechanistic support for UV protection for prevention of melanoma.

Evolution of mammalian CD1: marsupial CD1 is not orthologous to the eutherian isoforms and is a pseudogene in the opossum Monodelphis domestica

CD1 is a member of the major histocompatibility complex (MHC) class I family of proteins that present lipid antigens to T cells and natural killer (NK) T cells; it is found in both eutherian mammals and birds. In eutherians, duplication of the CD1 gene has resulted in multiple isoforms. A marsupial CD1 homologue was identified in a set of expressed sequence tags from the thymus of the bandicoot Isoodon macrourus. Southern blot and genomic sequence analyses revealed that CD1 is a single copy gene in both I. macrourus and a distantly related marsupial, the opossum Monodelphis domestica, which is currently the only marsupial species for which a whole genome sequence is available. We found that the opossum CD1 is located in a genomic region with a high degree of conserved synteny to the chromosomal regions containing human and mouse CD1. A phylogenetic analysis of mammalian CD1 revealed that marsupial CD1 is not orthologous to the eutherian CD1 isoforms, consistent with the latter having emerged by duplication after the separation of marsupials and eutherians 170-180 million years ago. The I. macrourus CD1 gene is actively transcribed and appears to encode a functional protein. In contrast, transcription of the M. domestica CD1 was not detected in any tissue and the predicted CD1 gene sequence contains a number of deletions that appear to render the locus a pseudogene.

Comparison between ultraviolet-visible and near-infrared elastic scattering spectroscopy of chemically induced melanomas in an animal model

The work reported compares elastic scattering spectroscopy (ESS) for diagnosis of pigmented skin lesions in two spectral regions: UV-visible and near infrared (NIR). Given the known strong absorption by melanin in the near-UV to mid-visible range of the spectrum, such a comparison can help determine the optimum wavelength range of ESS for diagnosis of pigmented skin lesions. For this purpose, four South American opossums are treated with dimethylbenz(a)anthracene on multiple dorsal sites to induce both malignant melanomas and benign pigmented lesions. Skin lesions are examined in vivo with ESS using both UV-visible and NIR, with wavelength ranges of 330 to 900 nm and 900 to 1700 nm, respectively. Both portable systems use the same fiber optic probe geometry. ESS measurements are made on the lesions, and spectral differences are grouped by diagnosis from standard histopathological procedure. Both ESS datasets show strong spectral trends with the histopathological assignments, and the data suggest a model for the underlying basis of the spectral distinction between benign and malignant pigmented nevi.

Animal models of melanoma: an HGF/SF transgenic mouse model may facilitate experimental access to UV initiating events

Cutaneous malignant melanoma, the most lethal of the skin cancers, known for its intractability to current therapies, continues to increase in incidence, providing a significant public health challenge. There is a consensus that skin cancer is initiated by sunlight exposure. For non-melanoma skin cancer there is substantial evidence that chronic exposure to the ultraviolet B radiation (UVB) (280-320 nm) portion of the sunlight spectrum is responsible. Experimentally, UVB is mutagenic and chronic UVB exposure can cause non-melanoma skin cancer in laboratory animals. Non-melanoma tumors in animals and in humans show characteristic UVB signature lesions in the tumor suppressor p53 and/or in the patched (PTCH) gene. An action spectrum or wavelength dependence for squamous cell carcinoma in the mouse shows a major peak of efficacy in the UVB. For malignant melanoma, however, the situation is unclear and the critical direct target(s) of sunlight in initiating melanoma and even the wavelengths responsible are as yet unidentified. This lack of information is in major part a result of a paucity of animal models for melanoma which recapitulate the role of sunlight in initiating this disease. The epidemiology of melanoma differs significantly from non-melanoma skin cancer. Intense sporadic sunlight exposure in childhood, probably exacerbated by additional adult exposure, is associated with elevated melanoma risk. Melanoma is also a disease of gene-environment interactions with underlying genetic factors playing a significant role. These major differences indicate that extrapolation from information for non-melanoma skin cancer to melanoma is unlikely to be useful. We summarize in this review the experimental information available on the role of UV radiation in melanoma and give an overview of animal melanoma models. A new model derived by neonatal UV irradiation of hepatocyte growth factor/scatter factor (HGF/SF) transgenic mice is described which recapitulates the etiology, the histopathology and molecular pathogenesis of human disease. It is anticipated that the HGF/SF transgenic model will provide a means to access the mechanism(s) by which sunlight initiates this lethal disease and provide an appropriate vehicle for derivation of appropriate therapeutic and preventive strategies.

Photobiology and genetics of malignant melanoma

The current state of knowledge of melanoma genetics is reviewed. Mutations in the tumour suppressor gene CDKN2A and in the oncogenes N-ras and H-ras seem to play the most important roles in the development and progression of malignant melanoma. Experimental studies to determine the role of ultraviolet (UV) light in the induction of melanoma have been hampered by a lack of suitable animal models. The commonly used laboratory animals are not susceptible to the induction of melanoma upon exposure to UV radiation (UVR) alone. Recent observations with four different animals have suggested, however, that UVR may be involved in the induction of melanoma. The most recent model consists of human skin grafted onto immunodeficient mice. To date, using this model, only the combination of UVB (280-320 nm) exposure and topical promoter treatment has led to the development of malignant melanoma. The wavelength dependency of the induction of melanoma has been established in the fish model Xiphophorus. The application of such an action spectrum to humans looks possible.

Photobiology of Monodelphis domestica

The gray, short-tailed opossum, Monodelphis domestica, has been used for photobiologic studies since 1984. The presence of a light-activated DNA repair pathway in the tissues of Monodelphis has been used to identify pyrimidine dimers in DNA as initiating events for a number of ultraviolet radiation (UVR)-induced pathologies of the skin and cornea. Furthermore, Monodelphis, unlike common laboratory rodents, is susceptible to the induction of melanoma by UVR alone.

Sunscreen effects on UV-induced immune suppression

In order to protect the public against the adverse effects of sunlight, the scientific, medical, and particularly the dermatologic community has promoted "safe sun exposure." This strategy includes sun avoidance whenever possible, wearing hats and other protective clothing and/or devices, such as sunglasses, and extensive use of sunscreens. Sunscreen efficacy is determined by measuring the ability of the sunscreen to block ultraviolet (UV)-induced erythema (sun protection factor or SPF), and most sunscreen formulations on the market, if used properly, are very good at preventing erythema and sunburn. How well sunscreens protect against the other adverse effects of sunlight, such as immune suppression, is not as clear. The purpose of this paper is to review and discuss the literature in this area, concentrating on some of the complications of determining how well sunscreens protect against UV-induced immune suppression.

Skin tumor development and keratin expression in different experimental models. Relation to inducing agent and target tissue structure

The applicability of the experimental skin carcinogenesis model for studies of tumor development was examined by exposing the skin of various mouse strains to different chemical carcinogens and UV radiation regimens, in order to analyze the development and progression of the neoplastic process and the role of differentiation markers such as keratins. In tumor-sensitive hairy NMRI mouse skin, the chemical carcinogen, 7,12-dimethylbenz(a)-anthracene (DMBA) induced an abnormal epidermal cell differentiation and structural irregularities associated with an altered keratin expression, as well as numerous papillomas and squamous cell carcinomas. A suboptimal dose of UVB irradiation increased the number of DMBA-induced benign squamous neoplasms. Low doses of benzo(a)pyrene resulted in mild epidermal alterations, but only in one tumor. High doses of UVB induced a large number of undifferentiated spindle cell tumors with few keratinpositive cells in NMRI mice, similar though fewer tumors in hairy, heavily pigmented C57BL/6 mice, numerous papillomas and squamous cell carcinomas in hairless hr/hr mice but only two papillomas in hairy, moderately pigmented DBA/2 mice while UVA exposure produced only two papillomas in hairless SKH-1 mice. In conclusion, the extent and type of skin tumor development depended upon the induction regimen: physical, chemical, dose and duration, as well as on the skin structure: pigmentation and adnexal development, all of which have to be taken into account when relating experimental results to human conditions.

S-100 immunoreactivity in melanomas of the South American opossum Monodelphis domestica

S-100 immunoreactivity was determined 1) in foci of melanocytic hyperplasia, 2) in naturally occurring, ultraviolet radiation-induced, and 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced primary melanomas, and 3) in metastatic melanoma lesions in the South American opossum Monodelphis domestica. Preneoplastic lesions of melanocytic hyperplasia contained scattered cells with S-100-positive nuclei. All primary melanomas, with the exception of a single DMBA-induced tumor, contained cells with S-100-positive nuclei. The pattern of S-100 reactivity in tumors varied from large foci of S-100-positive cells to scattered individual S-100-positive cells. Lymph node metastases were S-100 positive, but metastatic masses in internal organs were usually S-100 negative. Although S-100 reactivity did not distinguish preneoplastic lesions from tumors or benign melanomas from malignant melanomas, identification of metastatic tumor cells clearly demonstrated malignancy.

A new allogeneic model for metastatic melanoma

Metastatic melanoma cells, clonally derived from an affected lymph node of an ultraviolet-irradiated laboratory opossum, were allografted subcutaneously into suckling young, juveniles and adults to determine their tumorigenicity and metastatic potential. All injected 1- and 3-week-old suckling young survived well beyond weaning at 8 weeks. One died 12 weeks after injection from the effects of rampant metastatic involvement, while the rest were killed 13 to 26 weeks after injection. At necropsy, most animals showed extensive primary tumour growth, many showed metastasis to nodes and/or lungs, and in some there was dissemination to distant sites including liver and spleen. Animals injected as juveniles or adults rejected the allografts. Injection of allogeneic malignant melanoma cells during early postnatal development facilitates successful, long-term allografting and metastasis without concomitant immunosuppressive agents. Developmental lack of self-recognition (immunological immaturity) or induced tolerance may be responsible. This unique model system will be useful for further metastasis studies and may be valuable for investigations of novel antineoplastic therapies.

UV-induced skin cancer in a hairless mouse model

Ultraviolet (UV) radiation is a very common carcinogen in our environment, but epidemiological data on the relationship between skin cancers and ambient solar UV radiation are very restricted. In hairless mice the process of UV carcinogenesis can be studied in depth. Experiments with this animal model have yielded quantitative data on how tumor development depends on dose, time and wavelength of the UV radiation. In combination with epidemiological data, these experimental results can be transposed to humans. Comparative studies on molecular, cellular and physiological changes in mouse and man can further our fundamental understanding of UV carcinogenesis in man. This is likely to improve risk assessments such as those related to stratospheric ozone depletion, and to yield well-targeted intervention schemes, e.g. prescribing a specific drug or diet, for high-risk individuals.

Public education and cancer of the skin. What do people need to know about melanoma and nonmelanoma skin cancer?

Cutaneous melanoma (CM) and nonmelanoma skin cancer (NMSC) have a high chance for cure if detected in an early phase of development. Patients who have these tumors may now be treated in the outpatient setting with a minimum of discomfort, inconvenience, and cost. Most skin cancer deaths are caused by CM. Until recently, CM incidence in the United States has been increasing faster than any other potentially lethal cancer, attributable at least in part to aggressive case detection and greater public awareness about the significance of risk factors and early warning signs of evolving tumors, resulting in increased numbers of curable tumors. Most CMs are discovered by patients or close acquaintances. Most CM deaths are related to patient delay in seeking medical care. Patient delay is attributed mostly to lack of knowledge rather than to fear and denial. In the United States, primary prevention of CM and NMSC has focused on encouraging sensible sun-exposure behaviors, while secondary prevention consists of a yearly national campaign that promotes skin awareness and self-examination and free examinations to detect evolving tumors, sponsored by the American Academy of Dermatology and the American Cancer Society. More attention is needed to encourage timely consultation for evolving tumors and predisposing risk factors and to focus screening and surveillance efforts of those people at greatest risk. Public education must continue to promote personal responsibility in the intervention process to reduce the morbidity and mortality associated with CM and NMSC.

Ultraviolet radiation-induced skin lesions in laboratory opossums (Monodelphis domestica) exposed from the weanling stage

After repeated exposure to ultraviolet radiation, adult laboratory opossums (Monodelphis domestica) may develop hyperplasia and neoplasia of the shaved skin. We exposed opossums from the weanling stage (8-10 weeks after birth) and compared the incidence of lesions at designated time points with those observed following introduction of adults (around 5 months old) to the same irradiation protocol (125 J/m2 of UVB three times each week for up to 40 weeks). The overall incidence of hyperplasia and neoplasia among opossums exposed from the weanling stage was markedly lower than among animals first exposed as adults, and hyperkeratosis and sarcoma were not observed. Although freckling was widespread, cutaneous melanocytic nevi were rare among animals first exposed as weanlings; however, one animal without freckling developed malignant melanoma with presumptive metastasis to the spleen. The basis of the lowered lesion incidence among weanling-introduced animals is not clear; it may be that cutaneous immunity to ultraviolet radiation damage is more efficient in juveniles than in adults.

Genetic analysis of ultraviolet radiation-induced skin hyperplasia and neoplasia in a laboratory marsupial model (Monodelphis domestica)

Monodelphis domestica, the laboratory opossum, develops hyperplasia and neoplasia of shaved skin after repeated exposure to ultraviolet radiation (UVR). We exposed Monodelphis from genetically diverse families within our colony to determine whether there are any heritable components to the risk of two distinct skin lesion phenotypes-melanocytic nevus (MN) and advanced hyperkeratosis (HK). From about 5 months of age, animals were shaved and exposed three times a week to a dose of about 125 J/m2 of UVR (spectral peak, 302 nm; range, 280-400 nm). Of 33 sibships (151 individuals) that completed at least 30 weeks of the protocol, 137 completed 45 weeks. For genetic analyses, each animal was classified at 30 and 45 weeks as affected with MN and HK or not affected. Heritabilities were estimated using a variance decomposition approach. Susceptibility to MN showed no significant evidence for a genetic component at 30 or 45 weeks. In sharp contrast, susceptibility to HK was under virtually complete genetic control (heritability, 0.999; P < 0.001) at 30 weeks, and had a moderately high heritability (0.702; P < 0.001) at 45 weeks. We conclude that this model has great potential for identifying genes that confer susceptibility to UVR-induced skin lesions and for investigating environmental factors that may contribute to the increasing incidence of skin cancer in human populations.

Cell lines derived from ultraviolet radiation-induced benign melanocytic nevi in Monodelphis domestica exhibit cytogenetic aneuploidy

The gray short-tailed opossum, Monodelphis domestica, develops dermal melanocytic nevi (MN) after long-term chronic exposure to UVB (midwavelength ultraviolet radiation) alone. We developed cell lines from six UVB-induced dermal benign melanocytic lesion biopsies. One of the MN was determined histologically to be a benign melanoma (BM), whereas the remainder were benign melanocytic hyperplasias (MH). The cell lines were not tumorigenic when injected subcutaneously into athymic nude mice. Protein extracts prepared from these cell lines were analyzed electrophoretically on polyacrylamide gels and protease zymograms in preliminary attempts to identify protein and protease markers for pathogenesis. Cytogenetic analyses showed that half (three of six) of the MN cell lines exhibited aneuploidy involving extra copies of chromosomes 3, 5, 7, and/or 8. This result suggests that nonrandom aneuploidy can be an early event in chronic UVB induction of benign dermal melanocytic lesions. Karyotyping also showed a centromeric variant of chromosome 7 in some animals, which was confirmed to be constitutional. These Monodelphis cell lines will be valuable reagents for future studies of UVB-induced damage to mammalian skin.

Ultraviolet radiation-induced corneal tumours in the South American opossum, Monodelphis domestica

Chronic exposure of the South American opossum, Monodelphis domestica, to ultraviolet radiation (UVR) induced 154 primary tumours of the cornea in 152 eyes. Tumours developed gradually; frank neoplasia was preceded by non-neoplastic proliferation of corneal stromal fibroblasts (keratocytes) and extensive neovascularization. Histologically, the majority of tumours (134 of 154) appeared to be fibrosarcomas arising from keratocytes, but about 12 per cent of the tumours (18 of 154) had a highly vascular appearance, suggesting haemangiosarcoma. In two eyes, squamous cell carcinomas overlay mesenchymal tumours. Ultrastructural features of UVR-induced corneal tumours were consistent with tumours, and cultured skin fibroblasts expressed high content of messenger RNA for the intermediate filament vimentin; no cytokeratin messenger RNA was detected in these cells and cell lines. Based upon their light microscopic, ultrastructural, and intermediate filament biosynthetic characteristics, the majority of UVR-induced corneal tumours in M. domestica appeared to be fibrosarcomas. Haemangiosarcomas constituted a smaller proportion of the tumours, and squamous cell carcinomas were very rare.

Rising trends in melanoma. An hypothesis concerning sunscreen effectiveness

Incidence rates of melanoma have risen especially steeply since the mid-1970s. The two principal strategies for reduction of risk of melanoma and other skin cancers are sun avoidance and use of chemical sunscreens. Rising trends in the incidence of and mortality from melanoma have continued since the 1970s and 1980s, when sunscreens with high sun protection factors became widely used. Commonly used chemical sunscreens block ultraviolet B (UVB) but are virtually transparent to ultraviolet A (UVA), which makes up 90 to 95% of ultraviolet energy in the solar spectrum. Because sunscreens prevent erythema and sunburn, and inhibit accommodation of the skin to sunlight, their use may permit excessive exposure of the skin to portions of the solar spectrum other than UVB. If melanoma and basal cell carcinoma are initiated or promoted by solar radiation other than UVB, as laboratory data suggest, then UVB sunscreens might not be effective in preventing these cancers, and sunscreen use might increase the risk of their occurrence. Alternative explanations for the rapid rise in the incidence and mortality rates of melanoma, such as changes in patterns of recreational sun exposure, are discussed. Traditional means of limiting overexposure to the sun, such as wearing of hats and adequate clothing and avoidance of prolonged sunbathing, may be more prudent than reliance on chemical sunscreens.

Topical toremifene: a new approach for cutaneous melanoma?

The distribution of topically applied toremifene (0.5-1 mg/day for 5 days) in the ultraviolet B (UVB)-induced Monodelphis domestica opossum melanoma model was examined. The mean concentration of toremifene measured in the skin was 1200 nmol/g, or > 500 times that detected in any other tissues (blood, brain, liver, testicles, heart, uterus, eyes). In plasma, toremifene could be detected in only one animal of six (0.04 nmol/ml). Intraperitoneal administration of 0.5 mg toremifene daily for 5 days in three female animals resulted in a mean uterus concentration of 22.9 nmol/g, or 400-fold that achieved by topical administration of 0.5 mg/day in three other female Monodelphis (0.05 nmol/g). The cytostatic effect of toremifene was studied in three human melanoma cell lines and three experimental cell lines derived from UVB-induced melanocytic nevi in M. domestica. Toremifene had a cytostatic effect on all cell lines (50% growth-inhibitory concentrations, 5.8-9.6 microM). Topical toremifene administration yields high local concentration with minimal systemic distribution. In addition, toremifene has a cytostatic effect at achievable concentrations in a variety of melanomatous cell lines.

Specific chromosomal defects associated with ultraviolet radiation-induced cutaneous tumors in Monodelphis domestica (Marsupialia, mammalia)

Cytogenetic analysis of eight ultraviolet radiation (UVR)-induced cutaneous tumors and one spontaneously transformed fibroblast cell line of Monodelphis domestica showed that two of the tumor cell lines were of murine origin and that the remaining six marsupial tumor cell lines had hyperdiploid stemline numbers ranging from 21 to 31. Each tumor cell line showed structural and numerical abnormalities. The single transformed fibroblast cell line also showed a hyperdiploid chromosome number with structural and numerical defects. All M. domestica tumor cell lines and the fibroblast line showed a common structural abnormality: deletion of the short arm (p) of a chromosome 1. In some cell lines, the short arm of chromosome 1 was replaced by a translocation with the X chromosome. We suggest, based on the Giemsa-banding homology of chromosome 1p in M. domestica and human chromosome 6q involved in melanomas, that marsupial chromosome 1p may harbor tumor suppressor gene(s) that are associated with UVR-induced cutaneous tumors.

Identification of a transforming ras oncogene in an ultraviolet radiation-induced corneal tumor of Monodelphis domestica

Chronic exposure of the gray, short-tailed oppossum, Monodelphis domestica to ultraviolet radiation (UVR) induces mesenchymal tumors of the cornea. High molecular weight DNA samples from 6 UVR-induced corneal tumors were assayed for their ability to transform NIH 3T3 cells to tumorigenicity. NIH 3T3 cells transfected with DNA from 5 of the corneal tumors produced 14 tumors in nude mice. Cell lines were established from these tumors. DNA from 13 of 14 tumor cell lines contained repetitive opossum DNA sequences. Southern blot analysis revealed that DNA from 3 of 4 cell lines derived from tumorigenic NIH 3T3 cells transfected with DNA from a single oppossum tumor contained opossum Ki-ras oncogene sequences in addition to the murine Ki-ras gene. Northern blot analysis of mRNA from a mouse tumor cell line containing opossum Ki-ras gene sequences showed mRNA species identical in size to opossum Ki-ras mRNA, as well as murine Ki-ras mRNA species. These results suggest that an activated Ki-ras oncogene was present in one of the original opossum corneal tumors tested. Thus, activation of Ki-ras may play a role in the development of UVR-induced corneal tumors in Monodelphis domestica. Further characterization of ras oncogenes in these opossum tumors may provide information on the molecular mechanisms by which UVR induces corneal tumors in this species.